Project Summary This proposal addresses Provocative Question 1 ?What molecular mechanisms influence disease penetrance in individuals who inherit a cancer susceptibility gene?? Li-Fraumeni Syndrome (LFS) is a rare, inherited disorder that leads to a higher risk of breast cancer, soft tissue sarcoma, osteosarcoma, brain tumor (including glioma), adrenal cortical carcinoma, and other cancers. TP53, encoding the p53 tumor suppressor, is mutated in ~75% of LFS families. TP53 mutations in LFS patients occur in the coding sequence (CDS) and produce mutant p53 proteins that lack most or all normal tumor-suppressive functions and often confer oncogenic properties. Beyond LFS mutants, over 200 naturally occurring germline mutants of TP53 are known, yet only a few of them cause measurable perturbation of p53 function. Recent reports demonstrate that a single nucleotide polymorphism in a TP53 noncoding region predisposes carriers to multiple types of cancer including glioma, neuroblastoma, skin basal cell carcinoma, esophageal squamous cell carcinoma, prostate cancer, and colorectal adenoma. As such, cancer susceptibility of this p53 noncoding mutant does not strictly mirror that of p53 germline mutations in LFS patients. Specifically, this noncoding mutant confers no increased risk of breast cancer, the most common LFS tumor, whereas it shares similarity with LFS mutants in glioma predisposition. This p53 mutant is positioned uniquely among known cancer-susceptibility alleles in that it is noncoding and present at higher frequency (~1 in 50 in general populations, i.e., over 6 million Americans and 100 million people worldwide carry this mutant). We hypothesize that the moderate reduction of p53 activity by the noncoding mutant predisposes carriers to glioma but not breast cancer. In this study, we will employ mouse models and cell lines to ascertain the causative role of this p53 noncoding mutant in cancer by comparing it with an LFS coding sequence mutant. In Aim 1, we will determine whether the p53 mutant increases glioma development in mice. In Aim 2, we will determine the role of this mutant in mammary tumor development in mice and in human breast epithelial cell transformation. In Aim 3, we will dissect the mechanisms of the p53 PAS mutant in tumorigenesis in comparison with an LFS mutant using mammary stem cells and neural stem cells. Data generated from this study will reveal the biological and pathologic function of p53 germline mutants beyond its established role in LFS. This project has potentially broad and far-reaching significance in that our findings will reveal mechanistic causal connections between germline variation and tissue-specific cancer penetrance.